Looking at the application of this language, what is "impracticable or extremely difficult" to show as a loss by the landlord for your paying late? Civil Code 3302 takes all the mystery out of it:
§3302. Money Payable With Interest
The detriment caused by the breach of an obligation to pay money only, is deemed to be the amount due by the terms of the obligation, with interest thereon.
That interest is the "legal rate" of 10% per year, without compounding, or 1/3650 th of the monthly rent per day. On a $1000 monthly rent, that's 27 cents per day, or $1.92 per week. The word "deemed" in this statute is significant. It means that even if the landlord claimed personal injury by furrowing his brow and gnashing his teeth over your late payment, pain and suffering over his deep grief that he would have to cover the mortgage out of his savings, paper and processing costs from having to write you a 3-day notice to pay or quit, the LAW says he's only suffered and is only due the unpaid amount and the few paltry cents of interest. A $50 late fee for being 5 days late is NOT 10% interest, but 370% interest. Because the landlord's loss is "deemed" to be the unpaid rent plus the daily interest, it is neither impracticable nor extremely difficult to assess his actual losses. The defines his losses for him. Therefore, in light of Section 3302, it can never be impracticable nor extremely difficult to assess the landlord's losses, and his late fee will always be illegal, and void.
Even with Section 3302, which resolves the conflict by itself, what kind of losses would a landlord suffer from your late payment, anyway? Since the mortgage payment is typically not due until the 15th of the month, if you paid by the 15th, he could pay his mortgage without having to dip into his personal savings at the rate of 27 cents per day [in the above example], and he would suffer absolutely no losses. Since the current savings interest rate is and has been for 20 years far below 10% rate, the landlord's actual loss starting the 15th would be his actual interest loss on the $1,000, of about 3 cents per day. The bank could tell him how much that is, if he can't use a calculator, so that amount is not difficult to assess.